PROJECT SUMMARY/ABSTRACT DNA EXCISION REPAIR AND DNA DAMAGE CHECKPOINTS Nucleotide excision repair and DNA damage checkpoints are two major cellular responses to DNA damage that are essential for genomic stability. The goal of our research is to understand the molecular mechanisms of these systems and to use this information to develop cancer prevention and treatment strategies. To accomplish this goal we will perform the following experiments. Aim 1. Nucleotide Excision Repair and Skin Cancer Development as a Function of the Circadian Clock. Our studies on the regulation of nucleotide excision repair in mammals have revealed that excision repair exhibits high amplitude oscillation with a daily periodicity in most mouse tissues. We will analyze the daily variability of excision repair of UV photoproducts in skin and determine whether this variability is associated with carcinogenicity of UV light delivered at different times of the day. The results of this study will enable us to make specific recommendations to lower the incidence of skin cancer induced by sunlight and other UV sources. Aim 2. Biochemical Analysis of the UV-Induced DNA Damage Checkpoint. We will purify and characterize the proteins involved in ATR kinase-mediated DNA damage checkpoint response which is the primary checkpoint pathway activated by UV and UV-mimetic chemical agents. We will reconstitute this checkpoint system from highly purified components in vitro and analyze the coupling of nucleotide excision repair with the ATR checkpoint signaling pathway. Understanding this DNA damage signaling pathway should facilitate the design of novel chemotherapeutic approaches to treat cancer.